My phone is a Blackberry Curve 8320 that I bought from T-Mobile. It has never been unlocked or anything. So today I got a new text message and I looked at my phone and it said I was connected to the AT&T network. It said that for about five seconds, then it said searching for network... After that it went back to T-Mobile. I am just curious why something like that would happen. It doesn't bother me or anything, I am just asking fully out of curiosity. Thanks.

Here in Vegas my bb switches like that alot .
But you will not get charged for roaming . Tmo and ATT have reciprocal agreements and use eachothers towers to boost coverage .
If you are on ATT network with Tmo as your provider , you may not be able to access all of TMO's services , such as internet connectivity and in some cases voicemail . But it will switch back to TMO's carrier as soon as it gets a strong enough signal . If you are on a call when it happens , you probably will loose the call :(